Part of the drive to consolidate the War and Navy Departments was based on experience in WW2.

Prior to the consolidation the only person an Admiral and General both answered to was the President himself. Making joint operations, or even joint planing, let alone budgeting or similar all the ore contentious and competitive.

The US did not even have a formal joint chiefs board until faced with a united front by the British at the Arcadia Conference a few weeks after Pearl Harbor.

While the idea of a civilian head of the defense establishment was one we in part inherited from the British, who have long held mixed feelings about an army especially that did not answer to someone not also an active member. Hence why Britain had such posts as First Lord of the Admiralty, a Member of Parliament responsible for the smooth operation of the Navy whose day to day operation is then overseen by the officer First Sea Lord. And we can see then its importation to the US with what after a century or so of growth became the posts of CNO and Army CHief of Staff and their Service Secretary bosses, who now had a joint boss that could sort stuff out short of the President making every call.